Separation of olefins



Patented Mar. l, y1949 SEPARATION F OLEFINs Alfred W. Francis, Woodbury,

Socony-Vacuum Oil Company,

N. J., assignor to Incorporated, a

corporation of New York Application March 13, 1947, Serial No. 734,509

(Cl. ZEG-677) 14 Claims. l

This invention relates to a process for separation of olefins frommixtures thereof with saturated aliphatic hydrocarbons and isparticularly concerned with the recovery of normally gaseous olens fromaqueous solutions of silver salts prepared by scrubbing a mixture ofolens and parains with such aqueous silver salts.

It is well known that olens are selectively absorbed from hydrocarbonmixtures by aqueous solutions of certain metal salts such as the saltsof silver, copper, mercury and the like. The copper salts normallyrequire the presence of activating compounds which may be designated ascosolvents, for example, various amines, amides and other derivatives ofammonia. The copper salts also absorb carbon monoxide and are highlysensitive to a number of other contaminants. Further, solutions ofcopper salts which are suitable for the present purpose are in generalhighly viscous in nature and are therefore diflicult to use.

The present invention is concerned with an irnprovement on the processof separating and purifying oleiins by absorption in aqueous solutionsof water-soluble silver salts as typified by silver nitrate and silverfluoride. These solutions require no co-solvents, do not adsorb carbonmonoxide to any appreciable extent and are highly selective with respectto olens. The olens may be recovered from the silver salt solutions byheating or evacuation or by the extraction process disclosed in U. S.Patent No. 2,077,041 issued April 13, 1937, to Davis and Francis.Heating is objectionable in that elevated temperatures tend to inducereduction of the silver salts by the action of hydrocarbons and/or metalWalls of the apparatus. Evacuation is diflicult to control and is alsosubject to losses of silver solution at points of transfer and handling.

The process of the above-mentioned patent avoids many of thesedifficulties but still involves mechanical losses of the expensivesilver solution. According to the present process the olefins arerecovered from the aqueous solution by stripping with carbon dioxide ornitrous oxide, preferably under pressures about equal to the pressureprevailing in the absorption zone. By use of this expedient it becomespossible to operate the process while retaining the silver solution in aclosed circuit under substantially constant pressure which avoids lossesof silver. According to a preferred embodiment circulation through thesystem is induced by means of a magnetically operated pump which avoidsthe necessity of packing through which silver salt solutions may belost.

The carbon dioxide or nitrous oxide may be employed in either gaseous orliquid phase, depending upon the process and temperature prevailing andit may be noted that other inert compounds which are gaseous under theconditions of the process may also be employed.

When carbon dioxide is used in liquid phase, the process depends uponthe solubility of hydrocarbons in liquid carbon dioxide and it may benoted that the olens are completely miscible with liquid carbon dioxidein all proportions.

Gaseous carbon dioxide acts as a stripping agent in much the same manneras stripping steam is employed in conventional hydrocarbon fractionationoperations. For this purpose, other inert gases such as nitrogen,hydrogen and air may be used with similar result.

It has been found that carbon dioxide is inert to the various componentsof the system under consideration. It does not react with silver nitratein neutral or acid solution to give silver carbonate or any otherprecipitate even when the carbon dioxide is under high pressure or inliquid form.

Carbon dioxide is relatively insoluble in concentrated silver nitratesolution and it has been found that silver nitrate solutions strongerthan 50% dissolve less than 0.8% by weight of liquid carbon dioxide. Itmay be noted that water will dissolve 6.0% by weight of carbon dioxidewhen in equilibrium with liquid carbon dioxide. In general, very strongsolutions of the silver salts are preferred since the solubility ofolens in aqueous silver salts is roughly proportional to theconcentration of silver salt. Any concentration of a soluble silver saltgives some effect and the dilute solutions are operative. However, theconcentrated, preferably saturated solutions are greatly preferredbecause of the greater overall eiliclency obtained by the use of silversolutions having a high solvent power for oleiins.

Nitrous oxide is similar to carbon dioxide in the above'respects and itis to be understood that where carbon dioxide is mentioned herein,nitrous oxide is also contemplated as an equivalent thereof.

If a pure olefin is desired as the final product it is, of course,necessary to charge a relatively narrow fraction of the hydrocarbonmixture from which the olefin is to be recovered. Thus, the hydrocarbonmixture should be fractionally distilled to separate a narrow boilingcut containing only one olefin. For example, in the case of ethylene,the gas charged to the absorption stage may also contain methane,ethane, propane, hydrogen,

carbon monoxide and carbon dioxide. Sulphur compounds such as hydrogensulfide and mercaptans will react with the silver salt and should beremoved from the charged gas by caustic scrubbing. Acetylene also isundesirable and should be removed if present, as by selectivehydrogenation.

The solvent power of aqueous silver salts for olefins varies withpressure, concentration oi' the silver salt solution and mol fraction ofthe olefin in the gas. The effect of these factors is well illustratedby consideration of the solubility of propylene in aqueous silvernitrate solutions.

In 71% aqueous silver nitrate at 25 C., the solubility of propylene wasfound to be 8 volumes at atmospheric pressure and 185 volumes (reducedto atmospheric pressure) when using liquid propylene, vapor pressure11.5 atm., about twice as much as that expected (92 volumes) if thesolubility were proportioned to pressure. This amounts to 18.7 g.propylene in 100 g. of silver nitrate solution or 1.06 moles per mole ofsilver nitrate.

In 53% aqueous silver nitrate at 0 C. the solubility of ethylene wasfound to be 63 volumes at 40 atmospheres (just under the vapor pressure)and 200 volumes at 60 atmospheres (900 lbs.). The latter is 12% byweight of the solution or 1.37 moles per mole oi' silver nitrate.

The disproportionate gas solubilities noted in the above examples,though unexpected, are not unreasonable in view of analogousobservations on solubility of ammonia in water and carbon dioxide inorganic solvents reported in the literature. For example, in 1 g. ofwater at 20 C. are dissolved 1.5 g. ammonia at 4 atmospheres and 17 g.or over eleven times as much at 8 atm. (Seidell, Solubilities of Organicand Inorganic Compounds. D. Van Nostrand, New York, ed. III, vol. I, pp.1035-6). Similarly, carbon dioxide is nearly three times as soluble inbenzene or toluene at 40 kg. per sq. cm. as it is at 20 kg., even at 35C., which is above the critical temperature, 31 C. (Seidell, supra, p.234). These abnormal solubility isotherms are probably related to thedensities of the gas or liquid solute layers which are likewise Idisproportionate to the pressures in the neighborhood of the vaporpressures.

When the olen is not pure, the solubility in silver nitrate isapproximately proportional to the mole fraction rather than the partialpressure, as shown in the following table:

lgcllign Olean Diluent @251071170 Per cent by 0.945 Propylene wei"MISJ25 C. is 18.7% by weight in 71% silver nitrate and 9.6% in 39% silvernitrate solution.

A nearly saturated solution of silver nitrate is thus favored in orderto increase the capacity. In fact propylene combines with solidanhydrous silver nitrate to form an easily flowing liquid complexcontaining about 1.58 moles of propylene to one of silver nitrate. Itfreezes at 0 C. On re duction of pressure, it evolves propylenequantitatively, leaving crystals oi silver nitrate.

A higher extraction temperature is suggested by the rapid increase insolubility of silver nitrate with temperature, so as to get a highercapacity for oleiins. This is opposed by a slightly lower molar capacityof the silver nitrate, as illustrated by the solubility of propylene in71% silver nitrate at 25 C., 18.7% at 0 C., 21.2%; and at about 25 C.23%. Other objections to an elevated temperature are a slightlyincreased tendency toward contamination of the solution by corrosion ofthe vessel or by reduction; and the risk of local cooling which mightcause crystallization and plugging of a passage. Room temperature andabout 70% silver nitrate are recommended. Solution of olefin in silvernitrate evolves heat and raises the temperature slightly, but desorptionwould produce a slight refrigeration.

To test the possibility of unfavorable reaction of silver nitrate withcarbon dioxide under pressure, a thick glass tube was charged with 50%silver nitrate and solid carbon dioxide, sealed and warmed to roomtemperature to melt both layers, which remained as clear, colorlesslayers after shaking. This indicated no formation of insoluble silvercarbonate. .The increase in volume of the lower aqueous layercorresponded to a. solubility of about 0.8% CO2. A similar experimentwith water had indicated a solubility of about 6.0% by weight. On theother hand, carbon dioxide was found to be completely miscible withparaffin hydrocarbons at 50 C. and with olefins at still lowertemperatures.

The invention contemplates adsorption of olefins from a gas containingthe same in admixture with parafilns by contacting the gas with anaqueous solution of a silver salt, preferably under increased pressureand then stripping the enriched silver salt solution by intimatelycontacting the same with carbon dioxide in either liquid or gaseousphase. As an alternative, the invention contemplates stripping by theuse of inert gases generally. Preferably both the absorption andstripping stages are carried out by countercurrent contacting. In thecase of inert gases, including gaseous carbon dioxide, the gas shouldboil at a temperature substantially removed from the boiling point ofthe olefin. For example, in the recovery of ethylene from mixtures ofethane the process, in eifect, involves substitution for ethane of a gaswhich is more readilyseparated by distillation. Carbon dioxide isgreatly preferred because it is readily removed by washing with Water,thus dissolving the carbon dioxide which may then be recovered byheating or pressure reduction for return to the system. It is, ofcourse, generally desirable to dry carbon dioxide so returned in orderthat it shall not affect the concentration of the silver salt solution.

Referring now to the annexed drawings, there is shown apparatus forpractice of the present process in diagrammatic form. A gas mixture suchas ethane and ethylene free of acetylene obtained by fractionation ofcracked gases is scrubbed for removal of sulphur compounds andcompressed to a pressure of about 900 lbs. per square inch. Gaseousmixtures which can be liqulfled at room temperature are generallycompressed to a pressure sufficient to liquefy the same before chargingof the process. The compressed gas is supplied by line I to an absorberII through which it rises counter-current to a silver nitrate solutionadmitted by line I2. The olefin is thereby absorbed at the high pressurein the silver nitrate solution and the denuded gas, in this case ethane,is discharged by vent'l pipe I3 from the top of absorber II. Theenriched silver nitrate solution is withdrawn from the bottom ofabsorber II and transferred by pump I4 through line I to the top of thedesorber I6 through which it passes downwardly counter-current to astream of carbon dioxide supplied from line I1. The carbon dioxidestrips the absorbed olefins from the `silver nitrate solution and amixture of carbon dioxide and ethylene is taken off by pipe I8 forrecovery of the olefins. The lean silver nitrate solution is thenwithdrawn from the bottom of desorber I6 and returned to the top ofabsorber II by line I2.

This entire circuit including absorber Il, desorber I6, pump I4 and theconnecting lines together with such heat exchangers as may be requiredby the process are maintainedl under an elevated pressure about equal tothe pressure of the compressed feed gases. It will be understood thatthe process may be operated at a lower pressure, even down toatmospheric pressure, but any reduction in pressure involves acorresponding reduction in absorption efciency and the inventionpreferably involves the use of elevated pressures upwards of 600 lbs.per square inch and preferably upwards of 900 lbs. per square inch. Lossof the expensive silver nitrate solution through packings and the likeis preferably avoided by the use of some such device as a magnetic pumpto induce circulation without employing moving parts projecting throughthe walls of any part of the system. The pump may be any one of severaltypes, as is most convenient, the main requirements being (1) a pistonof magnetic material, such as iron, with a corrosion resistant surfacesuch as silver plating or a stainless steel sheath; (2) a cylinder ofnon-magnetic and corrosion resistant material, such as stainless steel;(3) a solenoid with intermittent current, or moving physically with areciprocating motion, surrounding the cylinder, to actuate the piston;and (4) two check valves, one of which may be in the piston, and one atthe end of the cylinder. One form is described in Ind. Eng. Chem. 34,937 (1942).

Other means for providing circulation without possibility of leakage canbe devised. It might be accomplished with thermal convection. Or thecirculating pump might be operated with a sealed induction motor. Or thestuffing boxes of a conventional pump might be sealed against leakage byglands with high pressure gas.

Returning now to the mixture of carbon dioxide and ethylene dischargedthrough line IB, this is transferred to a stripper I9 wherein themixture is washed with water to separate the carbon dioxide and yieldrelatively pure ethylene discharged by pipe 20. The solution of carbondioxide in water is transferred by line 2| from the bottom of stripperI9 to a flash tower 22 wherein the pressure is sharply reduced to evolvecarbon dioxide. Th carbon dioxide is compressed to the pressure of theabsorber-desorber system by compressor 23, cooled in heat exchanger 24and returned to the desorber through line I1. Such 6 drying of thecarbon dioxide as may be necessary can be carried out in the cooler 24or by the use of suitable deslccant chambers in the return line. Waterfrom the bottom of flash-tower 22 may be recycled to the stripper I9through pump 25, heat exchanger 26 and return feed line 21.

The pressures prevailing in the cycle including stripper I9 and ashtower 22 need not necessarily be closely controlled. However, since thisis a substantially closed cycle, it is convenient to operate the same atpressures approaching those prevailing in the absorber-desorber system.The pressure in the flash tower 22 must, of course, be substantiallylower than that in stripper I9 in order to obtain a good separation ofcarbon dioxide. Thus, the stripper I9 may be operated at a pressure ofabout 900 lbs. per square inch and the water-carbon dioxide mixture maybe flashed to 100 lbs. per square inch or less in the tower 22.

I claim:

l. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixturescountercurrent with a concentrated aqueous solution of silver nitrate athigh pressure and thereafter stripping the olefins from said aqueoussolution by contacting said solution countercurrent with liquid carbondioxide substantially at said high pressure and recovering olens fromsolution with said carbon dioxide by washing lwith water.

2. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixturescountercurrent with a concentrated aqueous solution of a water-solublesilver salt at high pressure and thereafter stripping the olefins fromsaid aqueous solution by contacting said solution countercurrent withliquid carbon dioxide substantially at said high pressure and recoveringolefins from solution with said carbon dioxide by washing with water.

3. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of silver nitrate and thereafter strippingthe olefins from said aqueous solution by contacting said solution withgaseous carbon dioxide.

4. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises'contacting said mixtures with aconcentrated aqueous solution of a water-soluble silver salt andthereafter stripping the olefins from said aqueous solution bycontacting said solution with gaseous carbon dioxide.

5. A process for separation of olefins from mixtures thereof lwithsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of silver nitrate and thereafter strippingthe olefins from said aqueous solution by contacting said solution withliquid carbon dioxide.

6. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of a water-soluble silver salt andthereafter stripping the olefins from said aqueous solution bycontacting said solution with liquid carbon dioxide.

'1. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with asubstantially saturated aqueous solution of silver nitrate andthereafter stripping the olefins from said aqueous solution bycontacting said solution with carbon dioxide.

8. A process for separation of oleiins :from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with asubstantially saturated aqueous solution of a watersoluble silver saltand thereafter stripping the olens from said aqueous solution bycontacting said solution with carbon dioxide.

9. Aprocess for separation of olefns from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with ayconcentrated aqueous solution of silver nitrate in an absorption zoneand thereafter stripping the oleflns from said aqueous solution bycontacting said solution with carbon dioxide in a desorption zone atabout the same pressure as that prevailing in said absorption zone.

10. A process for separation of olens from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of a Water-soluble silver salt in anabsorption zone and thereafter stripping the olefins from said aqueoussolution by contacting said solution with carbon dioxide in a desorptionzone at about the same pressure as that prevailing in said absorptionzone.

11. A process for separation of olens from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of silver nitrate and thereafter strippingthe olefins from said aqueous solution by contacting said solution withcarbon dioxide.

12. A process for separation of olefins from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixtures with aconcentrated aqueous solution of a water-so1uble silver salt andthereafter stripping the olefins from said aqueous solution bycontacting said solution with carbon dioxide.

13. A process for separation of olens from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixturescountercurrent with concentrated aqueous silver nitrate solution at highpressure in an absorption zone and thereafter stripping olens from saidsolution by contacting the same countercurrent -with an inert gas in adesorption zone maintained at about said high pressure prevailing insaid absorption zone.

14. A process for separation of olens from mixtures thereof withsaturated hydrocarbons which comprises contacting said mixturescountercurrent with concentrated aqueous silver nitrate solution at highpressure in an absorption zone and thereafter stripping olefins fromsaid solution by contacting the same countercurrent with nitrous oxidein a desorption zone maintained at about said high pressure prevailingin said absorption zone.

ALFRED W. FRANCIS.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Germany Mar. 10, 1932Number Number

